Recent investigations of the psychological and psychophysiological effects of caffeine have questioned the indiscriminant use of this commonly accepted drug. In some individuals, chronic excessive caffeine consumption leads to the development of caffeinism, a syndrome which includes increased anxiety, depression, frequency of psychophysiological disorders, and possibly degraded performance. However, despite evidence that there exists a significant subpopulation of caffeine consumers at risk for this syndrome, research to date has focused primarily on excessive caffeine use in clinical samples (e.g., hospitalized patients, or persons with previously diagnosed anxiety disorder), or has utilized experimental protocols that do not examine the effect of caffeine under ecologically relevant conditions. For example, research on caffeine has typically looked at the acute effects of caffeine administered in a laboratory situation, as opposed to examining the long-term chronic effects of ad lib caffeine consumption. The use of caffeine products and the development caffeine abuse has been recognized as a psychologically subtle and physiologically idiosyncratic process, yet little attention has been paid to either the idiographic nature of caffeine use, or the subtle yet compelling aspects of its longterm use at more moderate levels. To address these issues the proposed study has four main purposes: 1) Explore the relationship between ad lib caffeine consumption and indices of the caffeinism in a nonclinical sample via survey and correlational techniques. 2) Experimentally manipulate the daily caffeine consumption of persons who consume high levels of caffeine in order to observe concomitant changes in psychological and psychophysiological variables related to caffeinism. 3) Delineate the extent to which a reduction in one's usual ad lib consumption level produces a "withdrawal" syndrome clearly definable by its symptom pattern and temporal parameters. 4) Examine more closely the effect of long-term moderate ad lib caffeine consumption by including a group of moderate caffeine in the experimental phase of the study.